Default Save location of document in Cocoa document based application

I have created a Cocoa document based picture drawing application. I want that the default location of a new document created using my app in Save/Save As dialog should be in ~/Pictures/MyAppName/ directory.

How can I achieve this?

I tried more or less what Ole suggested below, but it doesn't work. Here is my implementation of prepareSavePanel. What am I doing wrong?

- (BOOL)prepareSavePanel:(NSSavePanel *)savePanel
{
    if ([self fileURL] == ni开发者_JAVA百科l) {
        //new, not saved yet
        [savePanel setExtensionHidden:NO];

        //set default save location        
        NSArray *paths = NSSearchPathForDirectoriesInDomains(NSPicturesDirectory, NSUserDomainMask, YES);

        if ([paths count] > 0) {
            NSString *userPicturesPath = [paths objectAtIndex:0];
            NSString *myDirPath = [userPicturesPath stringByAppendingPathComponent:@"MyAppName"];


            //create directory is it doesn't already exist
            NSFileManager *fileManager = [NSFileManager defaultManager];
            BOOL isDir;
            BOOL useMyAppDir = NO;
            if([fileManager fileExistsAtPath:myDirPath isDirectory:&isDir]){
                if (isDir) {
                    useMyAppDir = YES;
                }
            } else {
                //create the directory
                if([fileManager createDirectoryAtPath:myDirPath withIntermediateDirectories:YES attributes:nil error:nil]){
                    useMyAppDir = YES;
                }
            }

            if (useMyAppDir) {
                NSURL * myAppDirectoryURL = [NSURL URLWithString:myDirPath];
                [savePanel setDirectoryURL:myAppDirectoryURL];
            }
        }
    } else {
        [savePanel setExtensionHidden:[self fileNameExtensionWasHiddenInLastRunSavePanel]];
    }

    return YES;
}

In your NSDocument subclass, override -prepareSavePanel:

- (BOOL) prepareSavePanel:(NSSavePanel *)savePanel 
{
    // Set default folder if no default preference is present
    NSDictionary *userDefaults = [[NSUserDefaults standardUserDefaults] persistentDomainForName:[[NSBundle mainBundle] bundleIdentifier]];
    if ([userDefaults objectForKey:@"NSNavLastRootDirectory"] == nil) {
        NSArray *picturesFolderURLs = [[NSFileManager defaultManager] URLsForDirectory:NSPicturesDirectory inDomains:NSUserDomainMask];
        if ([picturesFolderURLs count] > 0) {
            NSURL *picturesFolderURL = [[picturesFolderURLs objectAtIndex:0] URLByAppendingPathComponent:@"MyAppName"];
            [savePanel setDirectoryURL:picturesFolderURL];
        }
    }
    return YES;
}